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	Lyman-alpha forest - Wikipedia, the free encyclopedia
		In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121.6 nm to shorter wavelengths (higher photon energies).
		The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that is roughly proportional to the distance between us and the source of light.
		Since neutral hydrogen clouds at different positions between Earth and the distant light source see the photons at different wavelengths (due to the redshift), each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum as observed on Earth.
	en.wikipedia.org /wiki/Lyman-alpha_forest (249 words)

	Damped Lyman Alpha Systems (Site not responding. Last check: 2007-10-07)
		The Lyman alpha (Lya) forest is an absorption phenomenon in the spectra of back-ground quasistellar objects (QSOs).
		This prediction was soon followed by the first observation of numerous dicrete absorption lines blueward of the lyman alpha emission line in QSO spectra It can be observed in the ultraviolet (UV) and optical wavelength range, from the local universe up to the highest redshifts where QSOs are found (currently z ~ 5).
		A small fraction of the lines in the forest are not caused by HI but belong to the UV transitions from several common metals or heavy metal ions (various ionization stages of C, O, Mg,Si, Fe,and Al are most frequently seen).
	www.macalester.edu /astronomy/research/fitih/dla.html (309 words)

	Active Galaxies and Quasars - the Lyman-alpha Forest (Site not responding. Last check: 2007-10-07)
		These are Lyman alpha absorption from foreground structures, in which the quasar light probes an otherwise invisible component of cosmic gas.
		When the launch of HST provided the first capability of measuring Lyman alpha at low redshifts to the required accuracy, it was found that a few of these absorbers remain in the local Universe.
		The strong and broad emission peak is Lyman alpha, which is almost chopped in half by the onset of the Lyman alpha forest in the high-redshift quasar.
	www.astr.ua.edu /keel/agn/forest.html (431 words)

	Alternate View--Analog
		In this column, I want to discuss the Lyman-alpha forest, a characteristic of distant quasars that has become a standard tool of astrophysics and that provides convincing evidence that the cosmological red shift is real and the quasars are as far away as their red shifts would imply.
		The Lyman series was first discovered in 1906 by Harvard physicist Theodore Lyman (1874-1954), who was studying the ultraviolet spectrum of electrically excited hydrogen gas.
		Lyman had to do his work using a vacuum chamber, because all of the light in the Lyman series is in the far-ultraviolet region and is strongly absorbed by air.
	www.analogsf.com /0303/altview.shtml (1865 words)

	Cosmology from the Lyman-alpha forest
		One of the main goals of research in cosmology is to find and exploit observable properties of the universe which can be computed theoretically and are sensitive to properties of the cosmological model.
		Some of the primary results are: (1) The transmitted flux probability distribution function (TFPDF), and the power spectrum and correlation function of the transmitted flux in the Lyα forest are determined from observed quasar spectra, and presented in tabular form to allow for comparisons with theoretical calculations and with other data sets.
		(4) The redshift evolution of the Lyα forest mean transmitted flux is used to infer the evolution of the intensity of the ionizing background that pervades the IGM.
	repository.upenn.edu /dissertations/AAI3015344 (397 words)

	Lyman alpha systems and cosmology
		The Lyman series is the series of energies required to excite an electron in hydrogen from its lowest energy state to a higher energy state.
		In Lyman alpha systems, the hydrogen is found in regions in space, and the source for the photons are quasars (also called qsos), very high energy light sources, shining at us from behind these regions.
		The Lyman alpha systems have deuterium in them too, and as these systems also have low amounts of metals (heavier elements), one might hope that they are measuring unprocessed or primordial deuterium.
	astron.berkeley.edu /~jcohn/lya.html (1527 words)

	Please Post
		The Lyman alpha forest is a result of absorptions of quasar continuum by the diffusely distributed and photoionized intergalactic medium, which traces the underlying density field on scales larger than the Jeans length.
		As such, the Lyman alpha forest offers a unique way to study the large-scale structure of the universe at a wide range of redshift.
		However, precision cosmology using the Lyman alpha forest remains a challenge because of statistical and systematic uncertainties that have not been addressed.
	www.physics.ucdavis.edu /Abstracts/ZhanAbstract.htm (148 words)

	Encyclopedia: Lyman-alpha forest (Site not responding. Last check: 2007-10-07)
		The Lyman series is the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron).
		A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies.
		An absorption spectrum is a diagram depicting the wavelengths of electromagnetic radiation absorbed by a material.
	www.nationmaster.com /encyclopedia/Lyman_alpha-forest (762 words)

	Redshifts are cosmological: the Lyman Alpha Forest and Arp - Bad Astronomy and Universe Today Forum (Site not responding. Last check: 2007-10-07)
		Redshifts are cosmological: the Lyman Alpha Forest and Arp
		When a spectrum is taken of a high-redshift quasar, it invariably shows a series of absorption lines blueward of the Lyman alpha line (this is the emission line of hydrogen where the electron drops from the 2nd to the 1st energy level, and has a wavelength of 1216 Angstroms).
		The cosmological interpretation of the Lyman alpha forest implies that the light is interacting with intervening material, mainly hydrogen.
	www.bautforum.com /showthread.php?t=3296 (3644 words)

	altvw116
		The ultraviolet light accompanying an electron jump from the n=2 to the n=1 orbit is called “Lyman Alpha” radiation, because it is the longest wavelength member of a series of photons (or spectroscopic “lines”) corresponding to jumps ending in the n=1 orbit.
		Therefore, the presence of the Lyman Alpha Forest provides evidence that the dark matter present in our universe must be mainly of the cold variety.
		The Lyman Alpha Forest can also be used to probe another characteristic of the early universe, the density of normal matter that it contained.
	mist.npl.washington.edu /AV/altvw116.html (1992 words)

	Abstract (Site not responding. Last check: 2007-10-07)
		We examine the dynamical evolution and statistical properties of the supernova ejecta of massive primordial stars in a cosmological framework to determine whether this first population of stars could have enriched the universe to the levels and dispersions seen by the most recent observations of the Lyman-Alpha forest.
		We evolve a lambda CDM model in a 1 Mpc3 volume to a redshift of z = 15 and add ``bubbles'' of metal corresponding to the supernova ejecta of the first generation of massive stars in all dark matter halos with masses greater than 5 times 105 solar masses.
		However, comparison to observations show that Pop III supernovae can be significant contributors to the very low overdensity Lyman-Alpha forest.
	qso.lanl.gov /meetings/meet2004/abs_norman.html (227 words)

	The Golden Age of Cosmology
		At a resolution of about 5400, the spectra shows what appears to be the Lyman Alpha emission line redshifted from its rest UV (1216 Angstrom) wavelength to an observed wavelength in the IR of about 1.33745 microns.
		the Lyman alpha emission line is shifted from the ultraviolet into the red region of the electromagnetic spectrum.
		Because the recessional velocities are a significant fraction of the speed of light, the relativistic Doppler equation is used to estimate the redshift of a quasar.
	www.markelowitz.com /cosmology.htm (1086 words)

	QSO Absorption Lines and Other Cosmological Probes
		Lyman limit systems: These are basically low density clouds of hydrogen which are in some partial or fully ionized state.
		This is known as the Lyman alpha forest as there are literally a forest of absorption lines that come from clouds located at different redshifts along the line of sight.
		Damped Lyman Alpha systems: These are very interesting as they represent relatively large column density systems which are needed to produce the strong damping wings on the observed absorption line profile.
	zebu.uoregon.edu /1997/ph410/l22.html (1555 words)

	[66.01] The Lyman Alpha Forest at High Resolution for 1.6 $<$ z $<$ 2.0 (Site not responding. Last check: 2007-10-07)
		We report on a study of the Lyman alpha forest at intermediate redshift using a sample of quasars observed with the Keck I telescope + HIRES spectrograph.
		Through Voigt profile decomposition of the forest, we search for changes in the physical properties of individual absorbers to distinguish between scenarios where dN/dz is driven predominantly by the evolution of bulk cloud properties as opposed to changes in the ionization state of the average cloud.
		We also combine the forest data with coverage of the C IV spectral region to explore the possibility that the strength of evolution in dN/dz is related to the presence of heavy metals.
	www.aas.org /publications/baas/v32n4/aas197/412.htm (281 words)

	Lyman Alpha Forest (Site not responding. Last check: 2007-10-07)
		There are clouds of gas between distant quasars and the Earth that absorb ultraviolet light at the wavelength of the Lyman alpha line of hydrogen at a wavelength of 122 nm.
		Note that if Arp were correct and quasars had a redshift much larger than the redshift due to their distance, then there should be a gap on the blue side of the Lyman alpha emission line before the absorption lines began.
		So if Arp were correct the Lyman alpha forest would have to be an intrinsic property of the quasar, which would be a very unlikely situation.
	www.astro.ucla.edu /~wright/Lyman-alpha-forest.html (390 words)

	Lyman-alpha forest -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-10-07)
		These absorption lines result from (Click link for more info and facts about intergalactic gas) intergalactic gas through which the (A starlike object that may send out radio waves and other forms of energy; large red shifts imply enormous recession velocities) quasars light has travelled.
		This means the intergalactic neutral hydrogen clouds at different positions between Earth and the light source see the photons at different (The distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave) wavelength (see also (Click link for more info and facts about redshift) redshift).
		Some physicists attribute it to the conservation of energy since if energy is conserved it simulates an accelerating expansion of the universe through the Einsteinian general time dilation effect.
	www.absoluteastronomy.com /encyclopedia/L/Ly/Lyman-alpha_forest.htm (191 words)

	Encyclopedia: Lyman alpha forest
		The Lyman alpha Forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121.6 nm to shorter wavelength (higher photon energies).
		This means the intergalactic neutral hydrogen clouds at different positions between Earth and the light source see the photons at different wavelength (see also redshift).
		The majority of astronomers attribute it to the hypothetical expansion of the universe.
	www.nationmaster.com /encyclopedia/Lyman-alpha-forest (266 words)

	[No title] (Site not responding. Last check: 2007-10-07)
		The simulations present a physical picture in which most of this absorption arises in diffuse intergalactic gas that is close to the cosmic mean density.
		According to this physical picture, it is simplest to view a Lyman-alpha forest spectrum not as a collection of ``lines'' but as a continuous, non-linear map of the diffuse intergalactic medium along the line of sight.
		Statistical properties of Lyman-alpha forest spectra can test theories of structure formation, and they lead to new constraints on the values of the fundamental cosmological parameters.
	www.phys.ufl.edu /~sergei/abc1 (146 words)

	THE LYMAN-ALPHA FOREST IN THE COLD DARK MATTER MODEL
		The agreement between the simulated and observed line populations suggests that our calculation provides a realistic general picture for the origin of the Ly forest, even if the cosmological scenario and numerical realization are not correct in all their details.
		However, the mean opacity of the simulated forest climbs steadily with redshift, and the number of lines above a specified column density increases with z when blending is not severe.
		This evolution is driven primarily by the increase in physical density with z, which raises the neutral fraction, and hence the opacity, of individual absorbers.
	www-hpcc.astro.washington.edu /old_content/papers/neal/lowcol/paper4.html (3204 words)

	Astrophysics Seminar (Site not responding. Last check: 2007-10-07)
		The absorption in high redshift quasar spectra by neutral hydrogen in the intergalactic medium, called the Lyman-alpha forest, provides a good probe of the density field of the Universe on scales of a few megaparsecs.
		I present the Lyman-alpha forest power spectrum derived from 3300 SDSS quasar spectra (a factor of 50 more than previous measurements).
		I describe the constraints on the power spectrum of the initial density perturbations obtained by comparing this observational result to the predictions of cosmological numerical simulations.
	www.physics.uci.edu /semcoll/other_seminars/abstracts/w04/040218.html (112 words)

	[ wu :: astronomy ] (Site not responding. Last check: 2007-10-07)
		Strewn throughout the universe are "trees" of hydrogen gas that absorb light from distant objects.
		These gas clouds leave numerous absorption lines in a distant quasar's spectra, together called the Lyman-alpha forest.
		Much remains unknown about the Lyman-alpha forest, including the real geometry and extent of the clouds, and why there are so many fewer clouds today.
	www.ocf.berkeley.edu /~wwu/astronomy/photos/02241998.html (107 words)

	Chris D Impey: Annual Report 1998
		Petry, C. Impey and C. Foltz published their analysis of the FOS spectra of the Lyman alpha forest seen in the individual images of 1422+231 to look for small scale structure witnessed by differences in the equivalent widths of the absorption lines.
		Morris (DAO) are continuing to investigate the sizes and topolgies of the Lyman alpha absorbers using intermediate-redshift quasar pairs and Hubble Space Telescope.
		The quasars are separated by 1.65 arcmin corresponding to a linear separation of 412 kpc (for H_0 = 100 km/s/Mpc and q_0 = 0.5) at z = 1.83.
	www.as.arizona.edu /steward/annrep98/Chris_Impey.html (1634 words)

	Marie E. Machacek (Site not responding. Last check: 2007-10-07)
		High resolution spectra of quasar light by the Keck Telescope reveal a complicated forest of Lyman alpha absorption lines that map statistically the distribution of low column density hydrogen at redshifts when structure was still rapidly evolving.
		These spectra also contain deeper, damped Lyman alpha features characteristic of high column density hydrogen and the presence of 'metal' lines, such as carbon, that yield information about early galactic halos and star formation.
		Her most current project uses high resolution numerical simulations of the evolution of structure in four of the most promising cosmological models to generate simulated Lyman alpha spectra whose statistical properties can be directly compared to recent observations.
	www.physics.neu.edu /faculty/machacek.html (486 words)

	Michael Rauch's Homepage
		If most of the gas seen from its neutral hydrogen absorption in the Lyman alpha forest were very tenuous the ionization could be extremely high, and an enormous amount of matter could be hidden in the form of ionizing gas.
		Damped Lyman alpha absorptions systems (DLAS) in QSO spectra get their name from the (Lorentzian) damping wings of the Lyman alpha line of neutral hydrogen visible in some very high column density QSO absorption systems.
		Moreover, many of the other properties of matter at high redshift (as observed, e.g., in the Lyman alpha forest or the Cosmic Microwave Background) agree better with a picture of hierarchical structure formation, where most high redshift galaxies are small and each modern galaxy is an agglomeration of multiple predecessors at high redshift.
	www.ociw.edu /~mr (2481 words)

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